SGD-OD: investigating the potential oxygen demand of submarine groundwater discharge in coastal systems

Submarine groundwater discharge (SGD) supplies nutrients, carbon, metals, and radionuclide tracers to estuarine and coastal waters. One aspect of SGD that is poorly recognized is its direct effect on dissolved oxygen (DO) demand in receiving waters, denoted here as SGD-OD. Sulfate-mediated oxidation of organic matter in salty coastal aquifers produces numerous reduced byproducts including sulfide, ammonia, dissolved organic carbon and nitrogen, methane, and reduced metals. When these byproducts are introduced to estuarine and coastal systems by SGD and are oxidized, they may substantially reduce the DO concentration in receiving waters and impact organisms living there. We consider six estuarine and coastal sites where SGD derived fluxes of reduced byproducts are well documented. Using data from these sites we present a semiquantitative model to estimate the effect of these byproducts on DO in the receiving waters. Without continued aeration with atmospheric oxygen, the study sites would have experienced periodic hypoxic conditions due to SGD-OD. The presence of H2S supplied by SGD could also impact organisms. This process is likely prevalent in other systems worldwide.


SGD-OD mass balance model
To highlight the potential impact of SGD-OD, we developed a simple model using the mass balance of electrons needed to reduce DO in a typical marine ecosystem.During oxygen reduction, molecular oxygen is converted to oxidized substrate and/or water through the gain of four electrons.Thus, to reduce the DO concentration by 100 µmol L −1 requires 400 e − L −1 .These electrons are supplied by the reduced byproducts (electron donors) carried by SGD.For example, to oxidize NH 4 + to NO 3 − requires a loss of 8 electrons (− 8e − ) and oxidizing 100 µmol L −1 of NH 4 + to NO 3 − results in an electron loss of 800 e − L −1 .We recognize that Fig. 1 represents a highly simplified version of more complex processes.For example, some electron donors are not oxidized rapidly and thus the true reduction capacity also depends on the timescale and the effects of microorganisms 5 .We assume the inorganic components (primarily H 2 S and NH 4 + ) oxidize rapidly, within a few days 10 .However, OM is comprised of an array of compounds that degrade at highly variable rates, with less complex OM, such as carbohydrates and proteins, often preferentially utilized over other more complex molecules such as lignin 11 .Labile OM, produced from fresh biomass, consumes DO more typically at a Redfield-type O 2 :C of 0.77 12,13 .In coastal systems OM consumption and biological oxygen demand (BOD) are most often dominated by dissolved organic matter (DOM) and is typically measured as the DO loss that occurs over a 5 to 28 day period 14 .While BOD rates vary significantly, consumption rates greater than 50% of DOC have been found in coastal systems influenced by urbanization 15 .The magnitude of this fraction likely differs depending on the environment.Hereafter, we conservatively assume 15% of the DOC in SGD is oxidized within a few days.This allows us to include DOC in the calculations without putting undue weight on this component.Since CH 4 in shallow systems evades to the atmosphere within a few hours 15 , and CH 4 oxidation rates can vary with salinity and temperature 16 , we reduce its potential effect by 60%.While this is a crude approximation, it does allow us to demonstrate the impact of CH 4 relative to other electron donors given this important by product 17 . (1) Figure 1.Illustration of reactions within the subterranean estuary that enrich submarine groundwater discharge (SGD) in electron donors.When these electron doners are transported to coastal waters by SGD, they react with oxygen, reducing the dissolved oxygen content of receiving waters.

SGD study sites and results
Fluxes of SGD to estuarine and coastal waters occur episodically.During tidal cycles, SGD usually peaks during falling tide 18 .Evidence for longer time scales is found in changing radium isotope signals [19][20][21][22] and in thermal records in continental shelf sediments 22,23 .The thermal records show short (1-3 day) episodes of strong SGD that are associated with wind patterns that temporarily lower sea level 22,23 .Storm events also produce strong episodic SGD [24][25][26][27] .Here, average fluxes and concentrations are used in the model, while recognizing that episodic fluxes may have more serious consequences.
To understand the potential impact of SGD on DO concentration, knowledge of the salty SGD flux is required.One of the most common techniques for constraining salty SGD is the use of radium isotopes 18 .To highlight the potential impacts of SGD-OD on receiving waters, we use site examples where SGD fluxes have been well characterized using radium isotopes and are accompanied by electron donor concentrations.A brief review of the use of radium isotopes in these studies is provided in the Supplemental Information (SI).We use component inventories to describe electron transfers in these shallow environments and to compare across SGD flux studies.An inventory is calculated as the total amount of a component in the water column over a given area.For example, if NH 4 + has a concentration of 35 mmol m −3 and is well mixed over a 2.5 m water column, the inventory is 87.5 mmol m −2 .

Okatee basin SGD fluxes
The Okatee River and salt marsh are located inland of Hilton Head Island in southeastern South Carolina, USA.This area was the site of the Land Use-Coastal Ecosystem Study (LU-CES) from 1999 to 2005, which studied the hydrography, hydrology, and biogeochemistry of the study area.During 2001-2002, hypoxic conditions (DO < 2 mg L −1 or < 64 µmol kg -1 ) were found in 21.5% of the observations in the estuary (n = 20,900) 28,29 .The Okatee River is formed where two small creeks converge in the upper reaches of the study area.The hydrology and biogeochemistry teams focused on the upper Okatee Basin (length = 5.6 km, low tide volume = 590,000 m 3 , tidal prism = 711,000 m 3 ).About 80% of the tidal prism that exits the upper Okatee returns essentially unmodified each tidal cycle, resulting in a residence time of 2-4 days 30 .
Monitoring wells were installed perpendicular to the river along two transects in the Basin and radium isotopes, nutrients, carbon, sulfide and Fe 2+ concentrations quantified.Moore et al. 30 estimated 226 Ra fluxes from this system and measured high concentrations of nutrients and carbon in Okatee groundwaters.Porubsky et al. 31 found significant correlations between NH 4 + , PO 4 3− , DOC, DIC, DON and 226 Ra in the groundwater.Fluxes can be calculated if the components are strongly correlated with 226 Ra and the flux of 226 Ra from the groundwater is known.We follow Porubsky et al. 31 who estimated mean fluxes of NH 4 + , DOC, and DON into the Basin (Table 2).They did not estimate H 2 S fluxes, so we used the correlation of H 2 S and 226 Ra (773 µmol dpm −1 , R 2 = 0.53) and the 226 Ra flux of 1.5 × 10 8 dpm day −1 to estimate a mean H 2 S flux of 116 kmol day −1 to the Basin.The concentrations of N 2 O, Fe 2+ and CH 4 were low and therefore not considered to impact DO consumption significantly.
Areal component fluxes per tidal cycle (mol m −2 tc −1 ) were estimated by dividing each component flux by the water area at high and low tide (Table 1).Each of the electron donor fluxes were multiplied by the electron exchange and the results were summed to determine the areal e − flux.The e − flux is dominated by H 2 S, with NH 4 + contributing another 10%.Both DOC and DON are minor contributors.Increasing the fraction of DOC oxidized would have a minor impact at this and other study sites.
DO concentrations in the Okatee River range from < 30 to > 300 µm L −127 .At steady state, the flux of electron donors to the Okatee River could consume 71 to 450 µmol L −1 of DO through each tidal cycle.This variability in potential DO reduction is primarily due to the differences in water coverage between high and low tide.www.nature.com/scientificreports/SGD is most pronounced during a falling tide 18 and component fluxes interact with a smaller volume of water over a smaller area.As a result, the reduction potential of DO at low tide increases by more than a factor of 6 (Table 1) and could completely deplete the DO if components are consumed on a tidal timescale of a few hours.
It is important to note, however, that SGD initially emerges at the start of ebb tide when there is more water in the system that dilutes the flux until absolute low tide.Furthermore, turbulence generated by exchange of the tidal prism in shallow water, in combination with river discharge, can rapidly entrain atmospheric oxygen into  www.nature.com/scientificreports/ the system.Buzzelli et al. 28 estimated mean DO utilization rates in the Okatee Basin of 0.15 mol m −2 day −1 .The estimated potential DO reduction at high tide of 0.19 mol m −2 tc −1 could reduce 2.5 times this DO given the tidal cycle.That the Okatee River experiences only episodic versus pervasive hypoxic conditions suggests that atmospheric exchange in this shallow, turbulent system must explain the difference.

Three sites near Sapelo Island, GA
Sapelo Island is a ~ 67 km 2 barrier island situated between mainland Georgia, USA, and the Atlantic Ocean.Its landward coastline is characterized by expansive salt marshes connected to the adjacent estuary through a complex network of tidal channels and creeks.In 2008, transects of PVC monitoring wells were installed across 3 salt marshes around Sapelo Island with 30 cm screened intervals, 1-5 m beneath the marsh platform.These transects stretched from the bank of a tidal creek, across a salt marsh, and up to an adjacent upland area.The sites were designated CI, HN, and PC and are described in detail by Schutte et al. 32 and in the SI.Samples for radium activity and concentrations of DOC, DON, NH 4 + , H 2 S, total Fe, and CH 4 were collected seasonally from each creek, ocean, and groundwater throughout 2008 and 2009.Schutte et al. 32 used these data to estimate SGD using a radium mass balance for each sampling period.Further, they used 228 Ra: 226 Ra mixing curves to identify the wells that tapped the sub-marsh aquifer and to indicate the wells most responsible for exchange with the tidal creek.They used this information to estimate the marsh component of SGD and multiplied SGD by the CH 4 concentration of the dominant sub-marsh aquifer to determine the SGD-driven CH 4 flux from the salt marsh.Here, we expand upon this work to determine the SGD-derived flux of other reduced constituents that may contribute to surface water DO demand (Table 2).
High tide total DO demand across all sites and time periods ranged from 4 to 110 µmol L −1 tc −1 with a median value of 34 µmol L −1 tc −1 .Low tide total DO demand was an order of magnitude higher, ranging from 48 to 1500 µmol L −1 tc −1 with a median value of 335 µmol L −1 tc −1 .The higher demand at low tide again reflects the order of magnitude lower volume of surface water in the estuary relative to high tide.There was between-site variability in total DO demand, with site HN having the highest demand and site CI having the lowest.However, there were no seasonal patterns in total DO demand either within or across sites.
The groundwater components that contributed most significantly to total DO demand were highly variable both spatially (CI, PC, and HN) and temporally (PC and HN).For example, at site HN, DOC contributed 41-45% of the total DO demand followed by H 2 S (36-43%).At site CI, NH 4 was dominant, contributing 70-72% of the total DO demand.At site PC H 2 S dominated (31-45%) DO demand followed by Fe 2+ (12-15%).

Mississippi coast
Mississippi Sound is an estuary located in the northern Gulf of Mexico along the coasts of Mississippi and Alabama, USA.Hypoxic events and fish kills occur frequently along the Mississippi coast, often in the western-most section [33][34][35][36] .A time series of five stations on the western Sound beaches were conducted between July 2017 and November 2019, where surface water and groundwater samples were sampled for radium, NH 4 + , DOC, CH 4 , and Fe 2+37 .Groundwater H 2 S samples were collected in January 2023 from each of the five stations.A mixing model using 228 Ra was constructed to determine the SGD flux into the Sound at each station per month, ranging from < 0 (i.e., seawater intrusion) to 62 cm 3 cm −2 day −1 , with an average SGD flux of 9.9 cm 3 cm −2 day −1 during summer and fall 37 .
Lowest DO values averaged 207 µmol O 2 L −1 during the summer and early fall.Based on temperature data, the average saturation of the water was 259 µmol O 2 L −1 , a reduction of 52 µmol O 2 L −1 , or 0.05 mol DO m −2 in a 1 m water column.This requires 0.2 mol e − m −2 .In surface waters, there is a trend of decreasing DO with increasing 226 Ra (Fig. 2) that qualitatively suggests that SGD is contributing to DO depletion.
Unlike the Okatee Basin and Sapelo Island, tidal influences were not directly measured and likely play a smaller role in SGD discharge in this system.The e -load to coastal waters was therefore calculated on a daily basis www.nature.com/scientificreports/using groundwater endmember concentrations collected from the 5 stations, the SGD flux, and the electron flux based on the number of electrons exchanged (Table 3).The e − supply from SGD was 0.63 mol m −2 day −138 .H 2 S clearly dominated the electron balance.On average, the total DO demand from SGD was 160 µmol DO L −1 day −1 , though the average DO concentration remained above 200 µmol L −1 .Continual aeration of these shallow waters appears to prevent DO concentrations from declining further.Atmospheric invasion of DO into the water column was calculated from the actual DO concentration, potential saturated DO, and the wind speed 39,40 .The average atmospheric invasion of DO into the water column was 373 µmol DO L −1 day −1 of O 2 into the shallow water column 39,40 .Turbulence could further increase aeration.During the summer and fall when fish kills were observed, the oxygen invasion exceeded 200 µmol L −1 day −1 .Therefore, oxygen resupply in this shallow environment offset the DO reduction due to SGD.Three fish kills were verified during this study even though DO at our sites did not approach hypoxic conditions, perhaps due to the high abundance of hydrogen sulfide (see "Discussion")..After eliminating other sources of radium, Peterson et al. 21concluded the enrichment must be due to SGD from aquifers on the continental shelf.They used the 228 Ra/ 226 Ra activity ratio (AR) in the samples to identify the source as an aquifer tapped by monitoring wells A and R located about 18 km offshore 21 .Using the average radium activities that had been measured in the wells, they determined that SGD from this aquifer could support an inventory of 1.7 m 3 m −2 of SGD in the study area.

Offshore South Carolina coast
Peterson et al. 21found average bottom water DO values changed at the end of Apache pier (4.5 to 6.5 m depth depending on tide) from 175 µmol DO L −1 (n = 8) prior to the discharge event on 4 August, to 102 µmol DO L −1 (n = 15) on 16-17 August, a reduction of 73 µmol DO L −1 .Taking the bottom water thickness as 2 m based on temperature profiles, this translates into a reduction of 0.15 mol DO m −2 , requiring 0.6 mol e − m −2 .Peterson et al. 21concluded that if the SGD contained no DO, dilution alone could explain the observed reduction in DO.
The composition of water in wells A and R was measured from 1999 to 2013.From these data (see SI), the average concentration (in µmol L −1 ) of each potential electron donor was used with the SGD inventory measured in August 2012 to calculate an e − inventory of 3.53 mol m −2 (Table 4).The inventory is six times higher than that needed to reduce bottom water DO concentrations by 75 µmol L −1 .Because the site is close to the surf zone located only 250 m from shore, waves and turbulence at the air-sea interface must constantly resupply DO through atmospheric exchange.
Another episodic offshore SGD event was sampled in August 2019 (Table 4) with samples spanning about ~ 170 km from Apache Pier to 10-20 km offshore of Charleston, SC 22 .This episodic event was predicted based on the wind field in late July 2019.Bottom waters were depleted in DO and enriched in radium, but not to the extent as measured at Apache pier in 2012.Figure 3 compares the 2012 and 2019 DO and Ra data.
In August 2019 the bottom waters off Charleston contained an average of 124 µmol DO L −1 .Unlike August 2012, the DO content of the bottom water immediately prior to the SGD event is unknown.We therefore use data from May 2019 when bottom water DO averaged 137 µmol L −122 .This implies a decrease of 13 µmol DO L −1 between May and August and a total e -demand of 0.46 e − m −2 for the 8.9 m bottom water column.Based on the radium isotope composition, wells A and R were again identified as the source of the radium enrichment and an SGD inventory of 0.26 m 3 m −2 was estimated 22 .This translates into an e − supply of 0.54 mol e − m −2 , more than sufficient to account for the reduction in DO observed over the time period (Table 4).Table 3. Calculation of the electron supply from SGD to the Mississippi Sound.*Based on SGD flux of 9.9 cm 3 cm −2 day −1 .**We assume 15% of the total DOC flux and 40% of the CH 4 is oxidized on a 2-4 day timescale.

Discussion
Hypoxia is recognized to be widespread, exerting significant biological stress in coastal ecosystems 1 .For example, coastal zooplankton and fish suffer several deleterious effects, including reduced prey capture efficiency, growth and reproductive potential, and even death 41 .Hypoxic zones further diminish and compress habitats, by making deeper, cooler waters unavailable in the summer 42 .Indeed, ecosystems exposed to long periods of hypoxia are characterized by low annual secondary production and minimal to no benthic fauna.Hypoxic zones in the Baltic Sea and Chesapeake Bay are estimated to have caused a loss in secondary production of ~ 6000 to 10,000 MT C annually; for the Gulf of Mexico, this loss may be as high as 17,000 MTC annually 1 .Increasing hypoxia also influences benthic biogeochemistry, both directly and indirectly.With the reduction in benthic fauna, sediment bioturbation is reduced, exacerbating the impacts of low DO on pore water chemistry, limiting oxygen-requiring reactions, e.g., coupled nitrification/denitrification, and stimulating loss of metals and phosphorus from sediments into the overlying water column.

Top-down vs bottom-up hypoxia mechanism
The classic top-down mechanism for generating coastal hypoxia first involves the supply of abundant nutrients to coastal waters, which increases biological productivity.The high productivity increases OM sedimentation to the seabed where this OM is oxidized by DO and drives hypoxia in stratified bottom waters that cannot receive new DO by mixing with surface or offshore waters.SGD can play a top-down role in DO depletion by supplying excess nutrients.Many studies have concluded that SGD nutrient supply is often greater than the supply from   local rivers 18,[43][44][45][46][47][48] .Alternately, SGD also plays an immediate bottom-up role in decreasing DO concentrations by supplying electron donors to bottom waters that reduce DO directly.Numerous papers suggest a link between SGD and low DO concentrations in estuarine and coastal waters [49][50][51][52][53][54] .Few papers link the reduced components in SGD directly to oxygen consumption in these systems.Peterson et al. 21demonstrated that simple dilution of bottom water with minimal DO SGD input could explain the DO reduction at Apache Pier in 2011.Guo et al. 55 measured SGD fluxes to the Changjiang (Yangtze) River estuary in China based on a 222 Rn mass balance model and found that SGD fluxes were higher in the summer when hypoxic conditions prevailed in the estuary and lower in the winter when hypoxic conditions were absent.They suggested SGD contributed to hypoxic conditions by either a top-down or bottom-up mechanism.Sanial et al. 9 applied a multi-tracer model to evaluate chemical mass balances in bottom waters along the coastline south of the Mississippi Sound and concluded that a common mechanism must supply Ra isotopes, Ba, and Si.After eliminating other sources, they concluded SGD must supply these components, driving oxidation of reduced SGD species (NH 4 + , H 2 S, CH 4 , DOC) and consuming DO in the process, thus promoting the development of seasonal hypoxia.Inverse correlations between the SGD proxy 228 Ra and DO in bottom waters further substantiated their hypothesis.
Here, we provide a mechanistic link between SGD and DO reduction and present supporting data from a variety of disparate coastal environments.These results further offer insights into when and where SGD-OD is most likely to influence coastal hypoxia.Estuaries experience quite different potential DO depletions at low and high tide due to extreme changes in water depth and areal extent.In the studies highlighted here, potential depletions at high tide ranged from 27 to 71 µmol L −1 .During low tide, potential depletions increased by an order of magnitude and would have resulted in anoxic conditions if no aeration occurred (Table 5).

Effects of H 2 S besides DO reduction
Specific components of SGD, such as hydrogen sulfide, which at pH < 7 is the most prominent sulfide species 56 , may also have direct detrimental effects irrespective of DO.The 96-h LC 50 H 2 S concentration for marine fish is 1.5 to 15 µmol L −157 .For example, throughout the year, water in the Okatee estuary has a pH ~ 7 31 , meaning about 50% of dissolved sulfide is H 2 S (the HS -ion is much less toxic than H 2 S) 57 .This water usually contains some measurable H 2 S with median concentrations much higher from April to September (hot) compared to October through March (cool) (Table 6).Here, the total dissolved sulfide concentration was divided by 2 to estimate the H 2 S concentration.The number of samples having > 1 µmol L −1 during the hot period was 11 out of 13, while in the cool period only 3 of 8 samples exceeded 1 µmol L −1 .A sample collected on 26 April 2002 was omitted, as it had 145 µmol sulfide L −1 and is viewed as an outlier.The average hot period H 2 S concentrations are above the lower limit of 1.5 µmol L −1 for 96-h LC 50 .These H 2 S concentrations will kill some organisms and likely stress many more.
Millero et al. 58 measured the half-life of H 2 S in air-saturated seawater at pH 8.0 to be 26 ± 9 h.However, Luther et al. 59 demonstrated that abiotic H 2 S oxidation rates yielded half-lives of hundreds of hours when conducted under oxic, clean, sterile conditions.Yet, when the sulfide oxidation rates were measured in the presence of chemolithotrophic microbes, light, and oxygen, they increased three orders of magnitude.The implication of these findings is that the residence time of H 2 S will largely be determined by the population size and activity of microbial communities rather than by chemical oxidants alone.www.nature.com/scientificreports/ We estimated an SGD flux of 0.084 mol H 2 S m −2 to the Okatee estuary during a tidal cycle (Table 1).Assuming a 3-day residence time for the water, a 2.7 m mean depth and an H 2 S concentration equal to 50% of the total dissolved sulfide, this SGD flux would produce an initial concentration of 93 µmol H 2 S L −1 before oxidation.With 93 µmol H 2 S L −1 as the initial concentration, a yearly median of 1.3 µmol H 2 S L −1 as the final concentration, and assuming first order kinetics, we determined a H 2 S half-life of 0.5 days.This is about 1 tidal cycle during which the initial H 2 S supply is exported with the tidal prism down the estuary.Given the half-life, only about 40% of this H 2 S is present in the return flow as about 80% of the tidal prism returns to the upper estuary without significant mixing during each tidal cycle 30 .Thus, only about 10% of the H 2 S that is introduced in the upper estuary is supplied to the lower estuary.Of course, there may be additional SGD further downstream.

Conclusions
Electron donors supplied by SGD significantly reduce the DO concentrations in estuarine and coastal waters.Of the systems considered, sulfide (HS − /H 2 S) was the most dominant electron donor in about half the cases.Ammonia (NH 4 + ), DOC or DON were dominant in the rest.Although the potential to deplete DO below hypoxic conditions was certainly present in many cases, reductions were counterbalanced by invasion of oxygen from the atmosphere in the shallower coastal systems.Our use of average fluxes and concentrations may have further underestimated the DO potential during periods of high SGD.
The subterranean estuary is expanding because of sea level rise and mining of freshwater aquifers 4 .This expansion increases the contact between sulfate and OM that has not been in contact with seawater for thousands of years.The byproducts of sulfate-OM reactions enrich the subterranean estuary in a variety of electron donors.As these electron donors are transported to estuarine and coastal waters by SGD, the potential for DO reduction is very likely to increase.More extensive studies of SGD-OD should therefore be conducted in stratified waters to better understand the role of SGD in direct DO consumption in coastal ecosystems.

Table 1 .
Calculations of the supply of electrons to Okatee River.The areal component flux, electron (e -) flux, DO reduction, and volumetric DO reduction are provided on a per tidal cycle basis at both high tide (H) and low tide (L).All values are the mean of all individual sampling events (n = 8) with sufficient data to calculate a groundwater flux.*Based on a 226 Ra flux of 1.5 × 10 8 dpm day −130,31 **High tide area = 7.13 × 10 5 m 2 .**Low tide area = 2.03 × 10 5 m 2 .***We assume 15% of the total DOC flux is oxidized on a 2-4 day timescale.

Table 2 .
Calculations of the supply of electrons to at the 3 Sapelo Island study sites (CI, HN, and PC).The areal component flux, electron (e − ) flux, DO reduction, and volumetric DO reduction are provided on a per tidal cycle basis at both high tide (H) and low tide (L).All values are the mean of all individual sampling events with sufficient data to calculate a groundwater flux at each study site (CI, n = 8, HN, n = 4, PC, n = 6).*High tide area for CI = 1.8 × 10 5 m 2 , HN = 4.9 × 10 5 m 2 , PC = 3.2 × 10 5 m 2 .*Low tide area CI = 1.1 × 10 4 m 2 , HN = 2.7 × 10 4 m 2 , PC = 2.7 × 10 4 m 2 .**We assume 15% of the total DOC flux and 40% of the CH 4 is oxidized on a 2-4 day timescale.

Figure 2 .
Figure 2. Dissolved oxygen versus 226 Ra at near shore sites in the western Mississippi Sound.Despite the shallow depth of sample collection (1 m), low concentrations of DO are associated with high 226 Ra activities along the Mississippi coast.Red circles denote samples collected during verified fish kills at the different sites.

Table 4 . 2 )
SGD components and their effects on DO off the coast of South Carolina measured during two SGD events.*Based on an SGD inventory of 1.7 m 3 m −2 .**Based on an SGD inventory of 0.26 m 3 m −2 .***We assume 15% of the total DOC flux is oxidized on a 2-4 day timescale.DO reduction (mol m −2 ) Water depth (m) DO reduction (µmol L −1 )

Figure 3 .
Figure 3.Comparison of August 2012 (black circles) and August 2019 (red circles) DO and 226 Ra off the coast of South Carolina.Data from 21,22 . ) Vol.:(0123456789) Scientific Reports | (2024) 14:9249 | https://doi.org/10.1038/s41598-024-59229-7 Data from a monitoring station on the northern South Carolina, USA coast at Myrtle Beach reveal episodic periods of hypoxic conditions in bottom waters in the late summer.The station is maintained by Coastal Carolina University at the end of Apache pier, which extends 250 m into the ocean.Surface and bottom waters are monitored continuously.The real-time and archived data from Apache pier are available online: http:// hydro metcl oud.com/ hydro metcl oud/ index.jsp.Radium samples collected from the end of Apache pier in mid-August 2012 contain the highest226Ra and 228 Ra activities ever measured in the South Atlantic Bight, exceeding measurements earlier in the summer by about one order of magnitude

Table 5 .
Comparisons of the estuarine and coastal systems.

Table 6 .
Cool-Hot weather comparison of H 2 S concentrations in the Okatee estuary.